The present invention relates to a stereophonic reproducing apparatus and, more particularly, to the stereophonic reproducing apparatus in which signals of each of the left and the right channels are vectorially added by means of a mixer circuit to thereby increase stereophonic sense.
In a conventional stereophonic reproducing apparatus, left and right sound signals are applied from a source of the stereophonic signals to respective left and right loudspeaker systems and are radiated as reproduced sounds independent from each other. This type of conventional stereophonic reproducing apparatus, however, has a disadvantage because the smaller the distance between the left and the right speakers is, the poorer becomes the stereophonic sense. Accordingly it has been proposed to enlarge the apparent distance between the left and the right sound sources by mixing the left and the right sound signals, which have heretofore been supplied independently to the left and the right speakers, with an appropriate ratio m by means of mixer circuits, producing a left sound signal A.sub.L1 =A.sub.L -m.multidot.A.sub.R and a right sound signal A.sub.R1 =A.sub.R -m.multidot.A.sub.L (wherein A.sub.L is the left sound input signal and A.sub.R is the right sound input signal, and the negative sign denotes simply a reversed phase), and supplying the left and the right sound signals thus mixed with each other to the left and the right speaker systems, respectively, for sound reproduction.
The principle of enlarging the apparent distance between the left and right sound sources will now be described with reference to FIG. 1. The left and the right sound signals A.sub.L1 and A.sub.R1 are applied as respective inputs to the left speaker 1 and the right speaker 2, which are spaced apart by the distance d. When the sounds radiated from the respective left and right speakers are listened to at a listening position 3, for example on the perpendicular bisector of the straight line connecting the centers of both the speakers (assuming that each of the left and the right sound sources and the listening position may be approximated by a point), the left sound pressure vector (P.sub.L -mP.sub.R) a and the right sound pressure vector (P.sub.R -mP.sub.L) b (where P.sub.L is the sound pressure produced by the signal A.sub.L, P.sub.R is the sound pressure produced by the signal A.sub.R, and a and b are unit vectors directed from the left and the right speakers, respectively, to the listening position (3) are added to produce a left signal sound pressure P.sub.L (a-mb) and a right signal sound pressure P.sub.R (b-ma). In other words, the stereophonic sounds are heard at the listening position 3 as if from the direction shown by broken lines, namely as if the sound sources were spaced apart by an apparent distance which is greater than the actual distance between the sound sources. Accordingly, by selecting an appropriate mixing ratio m, a satisfactory stereophonic sense is obtained even when the distance d between the left and the right speakers is relatively small.
However, the mere mixing of the left and the right sound signals in reversed phase as described above, in which the sound signals are mixed uniformly over the entire band, is disadvantageous because the sound volume is decreased in the low and the medium frequency ranges, the orientation of the sound source is from time to time varied in the high frequency range and a stereophonic reproducing apparatus for performing this kind of mixing operation, in which the left and the right sound signals to be mixed are obtained from the output of a pre-amplifier, must be provided particularly with a phase reversing circuit and, accordingly, necessitates additional costs.
Accordingly, an object of the present invention is to provide a stereophonic reproducing apparatus which is capable of obviating the above-described disadvantages such as reduction in sound volume in the low and the medium frequency ranges and variation in orientation of the sound sources in the high frequency range without any special phase reversing circuit which is, accordingly, less expensive, and is capable of mixing the left and the right sound signals in a reversed phase thereby increasing the stereophonic sense.